Ironically, I just answered a question about voice chat on Ask.fm, which is here:
QUESTION: Are you trying to make the game full PG? (Like even below PG-13) Couse it looks like you are trying to completely remove toxicity when there will always be people that act ughh... silly.Couse for example, voice chat would help immensly but your toxicity "excuse" ruins that idea...
League is a hardcore game, and it's played by many adults so no, we're not trying to make it a PG-13 game. In fact, we're not against offensive language per se. For example, saying "Fuck I missed that skillshot" or "Fuck, that was a bad engage, sorry" is completely fine and is not punished. You're only punished when you start focusing your language against someone else, for example, "Fuck you asshole, wtf was that play. Uninstll plz." Of course, hate speech, death threats and more severe language is not OK in any context, and that stuff will be punished with pretty much zero tolerance.
With regards to voice chat, we're not opposed to voice chat. A lot of research suggests that voice chat between friends is an awesome experience, but voice chat with strangers is more hit and miss. These are known problems with voice chat and these issues exist in every game. If we do voice chat, we'd like to try some new features to tackle these problems but right now, all our development teams are focused on other priorities.
To the OP though, there's a huge difference between 3rd party voice chat and integrated voice chat. Players that already have a voice chat solution installed, and are willing to add a stranger to their personal voice chat friends list and voice chat in a game are a very different, self-selected population compared to the general playerbase. In many cases, you could say these players are more likely to want to voice chat with others and very few people would do this purposely to yell at someone or be toxic to someone. If we had integrated voice chat, it would be available for everyone in the game, which has very different player behaviors.